It is known to provide various types of encoding for optical transmission systems. A particular field of application of such optical transmission systems is related to the introduction of 5G mobile systems which need higher capacity (100+Gbit/s) in the fiber transport network, starting from its first aggregation stages. Using dedicated fibers for backhauling radio base stations or for fronthaul connections between remote radio unit (RRU) and baseband processing unit (BBU) in a split architecture, is not always a solution, due to cable leasing or installation costs. Moreover, operators may use the same fiber infrastructure for multiple purposes, not only for mobile front-, mid-, and back-haul (i.e. X-haul) but also for fixed access and aggregation, further increasing the capacity requirement per installed fiber.
The trend towards network centralization and cloudification requires the capability to convey traffic from several non co-located RRU sites to one hub node, which hosts centralized network and baseband processing functions. This adds to the capacity requirement a distance requirement. Typical distances to be supported are up to 20 km, according to the maximum fiber propagation delay that the most common fronthaul protocols can tolerate (e.g. 100 μs, corresponding to 5 μs/km).
The capability to comply with any kind of network topology (bus, ring, tree, mesh) is also important to ensure the maximum deployment flexibility of future X-haul networks, enabling them to meet scenarios that can vary over a wide range, depending on operator and country.
DWDM networks can satisfy all the previous requirements. Coherent optical transceivers for transmission in Dense Wavelength Division Multiplexed (DWDM) optical transmission systems are known and can meet the distance requirement. However, the cost of coherent optical transceivers makes them less suitable for cost-sensitive network segments such as access and aggregation. New access technologies are increasing the traffic levels in access and aggregation networks segments, requiring higher optical channel capacities from 25 to 100 Gbit/s. There is a need for more cost-effective high speed optical transceivers.
A lower cost alternative to a coherent optical interface is a direct detection optical interface. Direct detection is widely used to provide 10 Gbit/s On Off Keying (OOK) optical channels. This technology is cheaper but suffers from two main drawbacks: (i) reduced sensitivity and noise tolerance; (ii) poor tolerance to chromatic dispersion. The first issue can be solved by using optical amplification, or by splitting the optical channels into two sub-channels at two different wavelengths, or by splitting the optical channels into two orthogonal linear polarization states. The second issue requires either the use of devices to compensate for the chromatic dispersion (e.g. Dispersion Compensating Fiber (DCF) or Fiber Bragg Grating (FBG)), or the use of a spectrally efficient modulation technique. For whatever modulation format, the narrower the spectrum, the lower the chromatic dispersion penalty. A narrow spectrum can be achieved by use of a multi-level modulation format or line coding. However, when using multi-level modulation formats, the achievable transmission distance is not always improved as the increased number of levels counterbalances the improved spectral efficiency, due to the lower tolerance of multi-level formats to the noise.
One typical solution provided by optical modules suppliers is the upgrade to DWDM of PAM-4 or DMT grey interfaces, (i.e. interfaces with no stabilized laser working in the 850 nm or 1310 nm wavelength regions) now used for interconnection purposes, which requires the minimal effort of replacing an uncooled laser with a stabilized one in the 1550 nm wavelength region. However, this solution is largely sub-optimal for system vendors or operators, for the reasons explained below.
DWDM uses the C band, centered on 1550 nm, rather than the O band, centered on 1310 nm. This has the big advantages of exploiting the EDFA amplification bandwidth and lower fiber attenuation values (attenuation coefficient is about 0.2 dB/km in C band and 0.3 dB/km in O band). However, moving from 1310 to 1550 nm, the fiber chromatic dispersion (roughly 1 and 17 μs/nm/km in O and C band, respectively), introduces a sensitivity penalty. At 10 Gbit/s the tolerated chromatic dispersion with 2 dB penalty is 800 μs/nm (about 47 km of fiber). Since the penalty scales with the square of the bit-rate, at 100 Gbit/s we obtain 8 μs/m for the same penalty value, which corresponds to about 500 m of fiber. Implementing DWDM systems in O band would not solve the problem because in absence of chromatic dispersion the four wave mixing (FWM, a fiber non-linear effect) would cause unacceptable inter-channel cross talk also at low channel power.
To deal with such chromatic dispersion in C band, non-coherent channels (e.g. based on 10 Gbit/s pluggable modules) rely on external dispersion compensators, which are modules that are placed in line to introduce a dispersion value equal and opposite to the fiber one. Coherent interfaces exploit instead electrical equalization at the receiver at the same purpose. In the former case, we have external devices that introduce additional cost and losses. In the latter one, energy consumption (tens of Watts) is the main drawback. Particularly for cost sensitive applications such as X-haul networks it would be desirable to avoid both of these issues.
Two known types of modulation formats at the transmitter side suitable for such front-haul interfaces are DQPSK and CAPS-3, which have benefits of good tolerance to chromatic dispersion. However, CAPS-3 has a better sensitivity and chromatic dispersion tolerance and significantly outperforms DQPSK for longer distances over about 15 km. For CAPS-3 the drawbacks are the cost and complexity of 8-state encoding circuitry, and the power consumption of the required high speed DAC. Accordingly, there is a need for a simplified encoding for optical transmission.